Various kinds of textile manufacture require the availability of fiber of uniform and relatively short staple length. Certain natural fibers such as cotton may inherently meet this requirement. Synthetic fibers, such as acrylic, polypropylene, nylon, polyester and the like, in the form of a tow of indeterminate length, typically require some type of processing in order to convert the tow to fibers of uniform staple length. The tow may have several hundred thousand individual fibers. In many cases, the tow is chopped into staple length fibers so that yarn can be spun. However, after being chopped, the uniform nature of the tow is destroyed and it is necessary to use a carding process to restore uniformity to the material.
As an alternative to chopping the tow, break drawing has been used to convert certain synthetic textile fibers in the form of continuous tow to a sliver of staple length fibers suitable for spinning into yarn. Break drawing is the practice of rapidly feeding the tow through two relatively closely spaced sequential pairs of rollers the second of which operates at a higher speed than the first so as to assert sufficient tension to break the fibers. For example, the output rollers may operate at a speed one and one-half to four times the speed of the input rollers, and the tow may be travelling at a speed of several yards per second. The rollers are conventionally spaced apart by a distance, e.g. three inches, just over the desired staple length, e.g. two and one-half inches. Rollers spacing as described herein refers to the distance between the nip points of the rollers. However, break drawing can be only used with synthetic fiber material which breaks under tension rather than simply stretching. Break drawing is also of limited use with synthetic fibers which require considerable force before breaking since such fibers may slip on either one or both of the input or output draw rolls, resulting in uneven staple length or undesired short-term variations in weight per unit length of the sliver so produced. As a result, only a few synthetic fibers such as acrylic can be processed using break drawing, while the majority of the synthetic fibers such as polyester, polypropylene, and nylon are either too elastic or are too strong for the break draw process. Even acrylic fiber tends to break unevenly and a further process of break drawing, sometimes called rebreak drawing, is frequently required to get the fibers to optimum staple length.
As a further alternative way of converting a synthetic fiber tow of indeterminate length into staple fibers of uniform length, it has been known to cut the fibers by passing the tow between a cutting roller and an anvil, the length of the staple fiber being determined by the pitch of the roller. One such system is sold as the TT12 Converter by N. Schlumberger and Cie of France. The cutting roller system inherently requires that the cutting roller be replaced when worn or when the staple length is to be changed.
Thus, it can be seen that there is a need for an apparatus and method which enable a wide range of types of continuous filament synthetic textile tow of indeterminate length to be converted into a sliver comprised of spinnable fibers of relatively short and uniform staple length and which eliminate the need for carding the short staple length fibers produced by such apparatus and method. More specifically, the invention recognizes a need for an easily controlled apparatus and method which incorporates both tensioning and localized heating of the fibers during break drawing so as to be able to produce from a tow of synthetic fibers of indeterminate length staple fibers of relatively short and staple length but without requiring the level of mechanical stress required to break the fibers at ambient temperature.
Since the illustrated and later described embodiments of the invention utilize a laser beam which is swept across the tow material in a transverse direction as the tow is drawn in a machine direction such that the interaction of the beam wavelength with the tow material causes localized heating of small fiber groups, mention is made of the prior art practice of using a laser for cutting cloth. Recognition is also given to the laser providing a unique source of coherent radiation capable of focusing all of the laser's output energy into an extremely small spot and thus achieving a high level of power density. The practice of sweeping a laser beam over a workpiece by use of a mirror for the purpose of using the interaction between the laser beam and the workpiece material to create localized heat and thereby modify the workpiece is also recognized. In such a system known as a Primary Pattern Generator (PPG) and described at pages 286-287 in the book "Lasers in Industry"(1972) edited by S. S. Charschaw, the workpiece is positioned under control of a computer which also controls the on-off status of the laser. However, so far as applicant is aware, it has not heretofore been known to use a mirror reflected laser beam to heat selected portions of a synthetic fiber tow of indeterminate length during its passage through a break drawframe and in a manner which results in the creation of staple fibers of substantially uniform and relatively short length.
With the foregoing in mind, the primary object of the invention is to provide an apparatus and method based on using known break drawing apparatus and the break drawing technique in conjunction with a concentrated source of heat, preferably produced by a laser beam, to convert continuous synthetic fiber tow to staple fibers of relatively short and uniform length.
Other objects will appear as the description proceeds.